Imaging process

ABSTRACT

This invention relates to a method and apparatus for creating high shear stresses in a particulate suspension prior to use of such suspension for imaging or inking. The method and apparatus in one environment function to stress a thin layer of an electrophoretic suspension of particles in a liquid carrier. The prestressed suspension is better suited than unstressed suspensions for preparing both monchromatic and polychromatic copies from originals by particle migration through the suspension when it is exposed to electromagnetic radiation in image configuration while in an electric field across the suspension between two electrodes. The method and apparatus is also used for printing inks in order to form a prestressed, uniform, thin layer of the ink for application to the press.

United States Patent [1 1 Zucker Sept. 3, 1974 IMAGING PROCESS PrimaryExaminer.lohn H. Mack Assistant Examiner-A. C. Prescott l t Ed Z k R tN.Y. [75] men or Wm oches er Attorney, Agent, or FirmJames J. Ralabate;Davld C. [73] Assignee: Xerox Corporation, Stamford, N.Y. P tr Richard ATomlin 22 Filed: an. 20 1971 1 J I 57 ABSTRACT [21] Appl" 108,213 Thisinvention relates to a method and apparatus for Related US. A li ti D tcreating high shear stresses in a particulate suspension [62] Divisionof Ser. NO. 764,721, 061. 3, 1968, Pat. No. to use of Such Suspensionfor imaging or inking- 3595772. The method and apparatus 1n oneenvironment function to stress a thin layer of an electrophoreticsuspen- 52] US. Cl. 204/180 R, 204/186, 204/189, sion of Particles in aliquid carrier T ore-stressed 2O4/3O0 suspension is better suited thanunstressed suspensions [51] Int. Cl B0lk 5/00 for Preparing bothmonchromatic and-polychfomatic [58] Field 61 Search 204/180 R, 300, 186,189 Copies from Originals by particle migration through the 1 suspensionwhen it is exposed to electromagnetic radi- [56] References Cited ationin image configuration while in an electric field UNITED STATES PATENTSacross the suspension between two electrodes. The method and apparatusis also used for printing inks in E335? "7 g order to form aprestressed, uniform, thin layer of the 3Z506I562 4 1970 coackleynl204/300 for applcancn to the Press' 4 Claims, 10 Drawing FiguresPATEN'I'EDSEP- 31w MEI 20F 3 FIG, 4

FIG. 6

PATENTEUSEP 31974 3Q833Q493 sum 30F 3 STORAGE TANK IMAGING PROCESS Thisis a division of application Ser. No. 764,721, now U.S. Pat. No.3,595,772, filed in the United States, Oct. 3, 1968. I

This invention relates in general to stressing particle suspensions forbreaking interparticle bonds and more specifically to a system forprestressing suspensions for better use in forming copies from anoriginal or in ink- A new imaging system in which one or more types ofphotosensitive radiant energy absorbing particles believed to bear acharge when suspended in a nonconductive liquid carrier, and placed inan electrode system and exposed to an image radiation configuration hasrecently been described. See US. Pat. No. 3,384,565 issued May 21, 1968in the names of V. Tulagin and L. M. Carreira. The particles in thissystem migrate in image configuration providing a visual image at one orboth of two electrodes between which they are placed. The system employsparticles which are photosensitive and which apparently undergo a netchange in charge polarity or a polarity alteration upon exposure toactivating radiation by interaction with one of the electrodes. Mixturesof two or more differently colored particles are used to secure variouscolors of images and imaging mixes having different spectral responses.Particles in these mixes may have either separate or overlappingspectral response curves and may be used in subtractive color synthesis.

If several colored particles are used in a system such as that describedabove, it is probable that these particles are of difierent sizes andirregular shapes. Because of this and otherreasons such as, for example,the fact that several particles may have different charges within theneutral mix, these particles tend to agglomerate together. 'In a colorsystem these agglomerates cause faulty color images since an irradiatedmigrating particle may pull with it another particle attached theretoand forming an agglomerate therewith which, otherwise, would notmigrate. The reverse may occur and even though a particle is struck byradiometric energy, it will not migrate because it is being held toother particles that are not irradiated byenergy striking that area ofthe suspension. The invention herein functions to break agglomerates,therefore providing better imaging in a system such as that described.

Also, the invention provides for improved inking methods in printingpress systems. In existing devices for printing presses many rollers arenecessary to meter out a uniform layer of ink to be placed on the pressfor printing of duplicates. The systems are complex and expensive butare necessary to get a uniformly dispersed, thin layer of particles heldwithin a suspension and forming the ink. By applying the inventionhereinto the inking system, the necessity for having many rollers iseliminated and a uniform,-thin layer can be controlled without thenecessity of an excessive number of rollers. The fluid dynamic forcesgenerated by the invention herein will disperse the particles uniformlyin the thin layer of ink. It may be that other systems exist or will bediscovered or invented that require in their operation suspensions thathave some or enough of the properties of the suspensions describedherein as being improved by this invention that this invention can beused thereon and such use is contemplated hereby.

Therefore, it is an object of this invention to improve methods andapparatus for breaking interparticle bonds in particle suspensionshaving agglomerable particles.

Another object of this invention is to improve the characteristics ofsuch suspensions in inking systems.

Still another object of this invention is to improve image quality incertain imaging systems.

A further object of this invention is to improve color separation inparticular color imaging systems.

Yet a further object of this invention is to provide relatively uniform,thin layers of suspensions while another object of this invention is toimprove image density in imaging systems.

These and other objects, features and advantages of the presentinvention are achieved by presenting certain external srresses to thesuspension which cause internal stresses within the particles, suchstresses being large enough to overcome the interparticle forces holdingparticle agglomerates together. These stresses include shear,compressive, and possibly others and are applied prior to use of thesuspension in systems requiring such material.

These and other objects and advantages of this invention will becomeapparent to those skilled in the art after reading the followingdescription taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic representation of an imaging system including onemeans of providing the necessary stresses;

FIG. 2 diagrammatically shows an imaging system with another means forpreparing the imaging suspensions;

FIGS. 3-7 are alternative means for preparing the imaging suspension;FIG. 8 diagrammatically shows an inking system with a means forproviding stresses on the ink;

FIG. 9 shows a force representation on an agglomerated particle withinthe suspension, and

. FIG. 10 schematically illustrates the forces on an elemental cuberepresenting a portion of an agglomerated particle as in FIG. 8.

, Referring now to FIG. 1, there is shown a transparent electrodegenerally designated 11 which, for illustration is made up of a-layer ofoptically transparent glass 12 over-coated with a thin opticallytransparent layer 13 of tin oxide commercially available under the nameNESA glass manufactured by Pittsburgh Plate Glass Co. This electrode isreferred to as the injecting electrode or imaging electrode. To becoated on the surface of the injecting electrode 11 is a thin layer offinely divided photosensitive particles dispersed in an insulatingcarrier liquid hereinafter referred to as the suspension. The termsuspension may be defined as a system having solid particles dispersedin a solid, liquid or gas. Nevertheless, the suspension described in thefollowing illustrations are of the general type including those having asolid dispersed in a liquid carrier. The term photosensitive .may bedefined as applying to any particle which, once attracted to theinjecting electrode will migrate away from it under the influence of anapplied electric field when it is exposed to activating electromagneticradiation.

Above the suspension 14 is a blocking electrode 16 which is connected toone side of a potential source 17 through a switch 18. The opposite sideof the potential source 17 is connected to the injecting electrode 11 sothat when the switch 18 is closed an electric field is applied acrossthe suspension 14 between the electrodes 11 and 16. An image projectormade up of a light source 19, a transparency 21 and a lens 22 isprovided to expose the suspension 14 to a light image of the originaltransparency 21 to be reproduced. The optical transparency of theelectrode 11 is shown by way of example and does not affect the scope ofthe invention herein, neither does the particular environment shown forimaging. Of course, this system of exposure is merely illustrative anddoes not materially affect the invention herein.

The electrode 16 is made in the form of a roller having a conductivecentral core 24 connected to the potential source 17. The core iscovered with a layer of a blocking electrode material 26 which may beTedlar, a polyvinyl fluoride commercially available from E. I. DuPont deNemours and Co. Inc.', or other material. In this and other embodimentsof the imaging system, the particle suspension is exposed to theimage tobe repro- 30 and then through the nip several times, and a portionpasses directly through the nip. The suspension,

whether forced through the intersection of the rollers or around theexit of the nip, encounters pressure and shear stresses. The stressedink then moves around a portion of the circumference of the rollersurface 26 imaged at the intersection between surface 26 and surface 13of the injecting electrode 11. The suspension not-used in the imageformed on electrode 11 is carried along the electrode 16 back to theposition beneath the suspension deposition means 32 to be mixed with newsuspension for further imaging. It may also be removed from the surface26 by any suitable means. An electrical bias that is negative relativeto the blocking elecduced while afpotential is applied across theblocking and injecting electrodes by closingswitch 18. Blockingelectrode 16 is caused to roll across me top surface of the injectingelectrode 11 with switch 18 closed during the period of image exposure.The exposure causes the exposed particles originally attracted to theelectrode 1 1 to migrate through theliquid and adhere to the surface ofthe electrode 16 leaving behind a particle image on the injectingelectrode surfacewhich is a duplicate of the original transparency 21. I3

' In the embodiment shown in FIG. 1, a stressing roller 30 is positionedto contact'the surface 26 of the blocking electrode roller 16 prior toimaging and a suspension deposition means 32 meters out the suspension14 on the surface 26 of the electrode 16. A doctoring rod 34 of glass orother suitable material causes the suspension passing beneath it to beof a somewhat uniform layer of acceptable thickness. The roller 30 isattached to a motor M-l which causes the roller 30 to rotate so that ithas rapidsurface velocitycompared to the velocity of the surface 26 ofthe blocking electrode 16. The roller 30 rotates in a direction shown bythe arrow although rotation in either direction is acceptable if the inkpasses through the nip.' Nevertheless, better results are obtained when'the roller rotates in the direction shown. The surface 33 of the roller30 may be made of conductive, semi-conductive or insulating material.One construction found to be useful and listed for illustrative purposesand not for limitation is an elastomeric layer on a metal core. Theroller 30 can be all metal, rigid or elastic, homogeneous ornon-homogeneous.

The stressing roller 30, the suspension dispenser 32 and the blockingelectrode roller 16 in FIG. 1 are mechanically connected together orhoused such that they will translate across the injecting electrode 11in a fixed relative relationship one to the other as the image is formedto the left side of the blocking electrode roller 16 (as illustrated).The suspension 14 is metered out of the suspension dispenser 32 andpasses the doctoring rod 34 according to the thickness requirement ofthe ink on the electrode 11. This is generally held to a thickness of 1mil. or less on the electrode 11 during imaging at the intersection ofthe electrode 11 and the electrode roller 16. y k

A portion of the suspension entering the nip between, the roller 30 andthe surface 26 of the roller 16 is carried completely around thecircumference of the roller trode 16 is preferably applied to thestressing roller 30 for reasons discussed hereinafter. This isaccomplished by connecting an electrical source 31 to the roller 30 or aconductive portion of the roller 30 if it has an insulating layer suchas a rubber coating 33.

FIG. 2 illustrates a different embodiment of a system to prestress theimaging suspensionprior to the actual imaging by the blocking electrode.The suspension is deposited on the injecting electrode 11 in anysuitable manner to insure a more or less thin layer of suspension.Preceding final imaging steps for the system, a roll '40 is rotated toeffect a relatively rapid tangential surface velocity compared to itstranslational velocity. Roll 40 is positioned to contact the suspensionon the electrode 11. This surface velocity is in the order of 40 inchesper second to 300 inches per second greater than the translationalvelocity of of the roller over the electrode. This causes the suspension14 to be stressed in the area formed between the electrode 11 andsurface of the roller 40. By placing a negative bias on the roller 40 byelectrical source 41, it tends not only to create pressure and shearstresses in the area of contact shown as the area 42, but it attractsunwanted positive pig'mentparticles' that would not be used in imagingif the injecting electrode is hole injecting and biased positiverelative to the blocking electrode 16. (More detail as to the imagingprocess is disclosed hereinafter.) The prestressed suspension is thenimaged by the electrode roller 16. A schematic driving system is shownillustrating a rack 35 with suitable gearing on the shafts of rollers 16and 40 to cause them to traverse the electrode 11. A bracket 36 is shownto move the rollers at the same rate and spacing across the electrode11. However, it may be desirable to move the rollers independently andeliminate the bracket.

Apparently, there is little difference in results between the systemhaving the stressing roller placed directly against the electrode 1 l onpreviously deposited suspension or on the blocking electrode 16 whichalso serves to meter out the suspension 14 used for imaging.

Referring now to FIG. 3 a system similar to that shown in FIG. 2 isutilized for prestressing and imaging of the suspension. Here, however,the apparatus to achieve the prestressing of the suspension is a member46 having a lower surface 47 angularly displaced from the electrode 11.The member should form a constricting channel with the electrode so thatthe suspension passing therebetween is subjected to external stresses.The surface may be mechanically biased toward the electrode 11 bysprings 48 and 49. The member is tiltable for adjustment of the angle ofthe converging surface 47 the electrode by independently maneuveringsprings 48 or 49 or any other suitable biasing means.

Motor M-2 causes translational movement of the member 46 over thesuspension 14 placed on the biasing electrode thereby developing certainpressure and shear forces in the area of the suspension trapped withinthe converging space between the surface 47 and the surface 13 of theelectrode 1 1. These shear and pressure forces act in a manner similarto those created by the rollers 30 and 40 in the previous figures toprestress the ink for improved imaging by the electrode roller 16 whichoperates in a manner substantially the same as that discussed inconjunction with the previous figures. A negative bias from anelectrical source 45 is shown in the member 46 to create a field betweenit and the positive bias placed on the electrode 11. This causes theattraction of positively charged particles within the suspension towardthe member 46. As ex plained hereinafter this permits improved imagingof the remaining particles'because of the field arrangement between theelectrodes 11 and 16. Because there is an electrical field betweenmember 46 and electrode 11, there may be air breakdown between itstrailing edge and the'electrode 11. To prevent this a lip portion 43 isadded to allow the gap to fill with suspension material rather than air.The angles shown in these figures are exaggerated for illustrativepurposes. Also, although the member 46 is shown as a wedge, it may beany shape as long as its lower surface can develop the proper stresseson the suspension.

' FIG. 4 shows other apparatus for achieving similar results to thosedescribed above by basically combining .the roller 40 of FIG. '2 and themember 46 of FIG. 3.

This combination uses a suspension prestressing roller 50 in combinationwith a member 52 shaped to accept the roller 50 in its lower surface 53and create stresses on the suspension flowing between it andelectrodell. The roller 50 joumaled for rotation about an axis 51rotates at a velocity substantially greater than its translationalvelocity across the electrode 11. This causes shear and pressurestresses similar to those caused by roller 30 in FIG. 1 and roller 40 inFIG. 2. Further the member 52 operating in much the same manner as themember 46 of FIG. 3 causes additional shear, pressure and electricalstresses as described in FIG. 3. Biasing springs 54 and 55 provide ameans for adjustability and control of the system. The imaging betweenthe electrode roller 16 and the injecting electrode 11 is the same asdescribed above. A lip 43 is added for reasons stated above.

FIG. 5 shows another means for accomplishing the stresses required byproviding a step wedge member 61 with a stepped lower surface 62. Allother aspects of this figure except for the lower surface shape are thesame as FIG. 3 above.

FIG. 6 shows another embodiment of specially shaped member 60operatively positioned at the electrode 16. The surface 61 of the member60 adjacent the electrode 16 is arcuate in shape in a manner determinedby the surface shape and dimensions of the electrode 16. The shapesshould be such that a converging channel is formed for suspensionflowing between the parts. Therefore, the distance between the member 60and the electrode 16 is greater at the ingress point of suspension flowthan at the egress point of the suspension flowing between the twoparts. The suspension may be metered out by a dispenser 32 and adoctoring rod 34 as has been described. The electrode 16 rotates in thedirection shown bringing the suspension 14 through the area between theelectrode l6 and the surface 61 of the member 60. The member 60 isstationary relative to the rotation of the electrode 16 but moves withthe electrode 16 as it traverses the imaging electrode 11. A bias isapplied between the member 60 and the electrode 16 as it was with thestressing roller 30 and electrode 16 in FIG. 1.

FIG. 7 shows an alternative meansfor applying stresses to thesuspension. Here a storage tank holds the suspension pending its use, e.g., for forming an image. A pump 72 moves the suspension through a line74 to a nozzle 76 having constrictions therein to cause a shear stressto the suspension particles as the suspension passes through the nozzle76. A flow control valve 78 is placed in the path of flow of thesuspension prior to its being deposited on the electrodes for imaging orto a dispenser. The control valve 78 determines the amount of suspensionmetered out and there is tubing to allow overflow suspension to returnto the storage tank to be recycled.

FIG. 8 shows a system supplying a prestressed marking suspension such asink for a printing press. Here a printing press with a plate cylinder 80is inked by an applicator roll 82 which must place a uniformlydispersed,

thin layer of ink suspension on the press for printing. It is importantto get a uniform dispersion and thin layer of ink in order to haveacceptable printing whether the copy is solid area, half tone,continuous tone or line representations. By using a very rapidlyrotating stressing roller 84 and a suitable metering means such asschematically shown by container 86 having an orifice 88 therein and adoctoring rod 89, the interaction at the nip between rollers 82 and 84causes a uniform dispersion of the particles in the carrier making anacceptable layer of uniformly dispersed particles of ink for the press.The thickness of the layer is determined by the contact pressure betweenthe two rollers 82 and 84, the relative surface velocity of rollers 84and 82, and theamount of ink metered through the orifice 88 of thecontainer 86 and past the doctor blade 89. An electrical bias isplacedon the gap between the rollers 82 and 84. The direction and magnitude ofthe bias depends on the particular inks used. The field will aid in thedispersing of the particles in the ink layer. The high speed stressingroller applies the ink electrophoretically since a field is maintainedbetween the stressing roller and the electrode roller during operationfor preferred results. The combined application of the suspensionelectrophoretically with a very rapidly overdriven stressing roller is anovel means of inking many types of machines and apparatus with a widevariety of suspensions or inks. This method of inking is usable on suchmachines as printing presses. The high cyclic speed of the inking rolleraverages the imperfections in roller surfaces as well as other machineimperfections and interrelations of parts. The improvement is in such amanner that a relatively uniform layer of ink is produced in the nipbetween the high speed roller and an applicator roller positionedadjacent the surface of the high speed roller. This method of applyingink will give a uniform dispersion of a layer of the ink whilecorrecting for such defects as eccentricitywithin tolerance levels. Thisapparatus can be used as a substitute for the multiple rollers necessaryto apply a uniform layer of ink to printing presses which currentlyexist in the art.

FIG. 9 schematically illustrates a greatly enlarged microscopicagglomerate of particles held within a suspension. The arrows on thefigure indicate the stresses acting upon this agglomerate of particles.The particles may be the same material, size or charge or different inany or all of these parameters. They may represent the three colors usedin subtractive color synthesis. When a shear stressingforce and acompressive force are applied to the agglomerate, theshearforces causeinternal stresses within the particles and the agglomerate itself andcause a fracturing along the weakest lines thereof. The weakest portionsof the agglomerates are generally the interfaces of the individualparticles. Therefore, the fracturing tends to occur between theparticles within the agglomerate causing each particle to free itselffrom the others and remain in an independent state ready to be actedupon by other forces such as the electric field and electromagneticradiation during imaging. The fracturing may occur from shear or tensilstresses on theparticleeven though only shear, compressive andelectrical forces are placed on the suspension.

Although the shearing of the suspension is illustrated as immediatelypreceding imaging inthe several embodiments illustrated, it has beenexperimentally shown that an hiatus of at'least several hours and evenconsiderably longer can intervene between the action of the shear stressmeans and the imaging of the suspension without recombination of thefractured agglomerates occurring.

OPERATION OF IMAGING SYSTEM A detailed description of the operation andtheories relating to the actual imaging system improved by thisinvention and discussing the interaction of the particles in thesuspension with the electrodes 11 and 16 is disclosed in US. Pat. No.3,383,993 issued on May'2l, 1968 in the'name of S. Yeh. The systemtherein described and the one used here as an imaging embodiment forthis invention provides an electric field across the imaging area suchthat the electrode 11 is positive and the electrode 16 is negative withrespect to each other. Therefore particles within the suspension thatare negatively charged will attract themselves to the positionhole-injecting electrode 11.

When the switch 18 is left in the open position as shown in the figuresand no potential is applied across the electrodes 11 and 16, thesuspended particles merely assume random positions in the carrier.However when the switch 18 is closed, thereby rendering the conductivesurface 13 of electrode 11 positive with respect to the surface 26 ofthe blocking electrode 16, negatively charged particles within thesystem tend to move toward the electrode 11 while any positively chargedparticles in the system would move toward the blocking electrode 16.This,'of course is without the influence of any exposure radiation toform an image between the two electrodes. y

The same phenomenon with the positive and negative particles occurs withrespect to the stressing roller 30 and the blocking electrode 16 inFlG.1 or the stress forming mechanism the electrode 11 in the other figuresshown. The stressing mechanism is biased nega-,

tively compared to the electrode surface with which it interacts.Therefore, the positively. chargedparticles plied electric field untilthey are subjected to exposure to activating electromagnetic radiation.The particles bound on the surface of the injecting electrode 11 make upthe potential imaging particles for the final image to be reproducedthereon. When activating radiation strikes the particles, it is absorbedby the photosensitive particle and makes the particle conductivecreating hole-electron pairs of charge carriers which may be consideredmobile in nature. These newly created'hole-electron pairs within theparticles are thought to remain separated before they can combine due tothe electrical field surrounding the particle between the twoelectrodes. The negative charge carriers of these hole-electron pairsmove toward the positive electrode 11 while the positive charge carriersmove toward the electrode 16. The negative charge carriers near theparticle-electrode interface at electrode 11 can move across the veryshort distance between theparticle and the surface 13 leaving theparticle with the net positive charge after sufficient charge transfer.These net positively charged particles are nowrepelled away from thepositive surface of electrode 11 and attracted to the negative blockingelectrode 16. Accordingly,-the particles struck by activating radiationof a wavelength with which they are sensitive, Le, a wavelength whichwill cause the formation of hole-electron pairs within the particles,move away from the electrode 11 to the electrode 16 leaving behind onlyparticles which are not exposed to sufficient electromagnetic radiationin their responsive range to undergo this change.

Consequently if all the particles in the system are sensitive to onewavelength of light or another and the system is exposed to an imagewith that wavelength of light, a positive image will be formed on thesurface of electrode 11 by the subtraction of bound particles from itssurface leaving behind bound particles in unexposed areas. If all thepolarities on the system are reversed, electrode 1 1 will preferably becapable of accepting injected holes from bound particles upon exposureto light and electrode 16 will be a blocking electrode incapable ofinjecting holes into the particles when they come into contact with thesurface of this electrode. The system may be operated with dispersionsof particles which initially take on a net positive charge or a netnegative charge and even with systems where the particles in thesuspension apparently take on both polarities of charge. In this latterinstance, the bias placed on the stressing mechanism will help removethe particles of unwanted polarity from the imaging system.

Typical voltages employed in the system for imaging while exposing maybe in a range of 300 to 5,000 volts.

Depending on the particular use to which the system is 'to be'put, thesuspension 14 may contain one, two,

atively immaterial as long as it shows response in some region of thespectrum which can be matched by a convenient exposure source. Inpolychromatic systems, the particles may be selected so that particlesof different colors respond to different wavelengths in the visiblespectrum thus allowing for color separation. Regardless of whether thesystem is employed to produce monochromatic or polychromatic images itis desirable to use particles which are relatively small in size becausesmaller particles produce better and more stable dispersions in thesuspension and are capable of forming images of higher resolution thanwould be possible with particles of larger size.

When the particles are suspended in the carrier, they may take on a netelectrostatic charge so that they may be attracted toward one of theelectrodes in the system depending on the polarity of its charge withrespect to that of the electrodes. Some of the particles in thesuspension may be positive, others negative and some even bipolar. Thewrong polarity of particles of the suspensions may affect the imagingcapabilities of the system by masking some of the right particles, or byagglomerating with some of the right particles in the system beforeimagewise particle migration takes place. In other words, the abovebehavior causes a portion of the suspended particles to be removed fromthe system as potential image-formers while others stay behind.

By electrically biasing the stressing mechanism, particles of the wrongpolarity can be removed fromthe suspension or at least substantiallyseparated from the electrode contacted by the stressing mechanism. Thestresses break the agglomerated particles and then the bias takeswrong-charged particles out of the system prior to imaging. If thestressing mechanism is not biased, it will nevertheless still functionto break up agglomerates, but without the additional benefit of removingparticles that cannot image in the system,

Consider a three color subtractive system where the suspension containsindividual particles of a magenta, yellow and cyan color which aresensitive to green, blue and red wavelength radiation respectively.Assumethat green light exposes the electroconductive glass processingthe tri-mix (suspension with the three color particles mentionedabove).Under optimum conditions the magenta particles absorb the light whilethe cyan and yellow particles reflect the light. The particles madeconductive by absorbing the radiation exchange charge with theelectroconductive glass as mentioned above. The magenta particles, as aresult of their activation by exposure to the green light, becomepositive and migrate away from the injecting electrode. The cyan andyellow particles remain generally insulating and are not affected sincethey have little or no photosensitivity in the green light range of thespectrum. The magenta particles migrate selective as made conductive tothe negative blocking roller or electrode. Thus, a color reproduction ofthe original green light is obtained in image configuration on theinjecting electrode.

by subtractive color. That is, the cyan and yellow pigments remainingappear green when viewed.

THEORY OF OPERATION One of the limitations in effective color separationby the photoelectrophoretic technique just described is theagglomeration of different color pigment particles to each other. Theinvention described herein breaks cant compared to the electrostaticforces operating on the overall suspension, and are responsible forparticle packing within the suspension because of the close proximity ofthe pigment particles.

In a polychromatic system, the agglomerations, which result because ofthese particle packings and because of other reasons such as ballmilling during pigment preparation, consist of multi-colored pigmentparticles with an overall net negative or net positive charge. When theagglomerate behaves as a unit instead of discretely, an error in colorseparation results because the non-illuminated particles migrateerroneously with the illuminated particles, or the illuminated particlesfail to migrate because they are bonded to the non-illuminated pigmentparticles.

In a monochromatic system the particles will migrate if energy of anywavelength within the panchromatic spectrum of the particle responsestrikes the particle. It is believed that energy absorption and responsethereto by the particles relate to the surface area of the particleavailable to be struck. The area-to-mass ratio of these particles isdecreased if several of them agglomerate together. Therefore greateramounts of energy are required in order to cause the migration of theagglomerated particle then would be necessary to cause the migration ofthe individual particles not attached to each other.

Once the magnitudes of the bonding forces internal to the agglomeratesare determined, the critical stresses necessary to overcome theseforcesin order to affect structural failure must be generated. This isaccomplishedby increasing the external state of stress thereby causingthe internal state of stress within the particle agglomerates to exceedthe stess level necessary to cause agglomerate particle failure. Bycausing this structural failure particle separation and therefore betterimaging can be effected.

The forces external to the agglomerated particles exerted by thestressing mechanism operates in the contact zone between the surface ofthe stressing mechanism and the electrode upon which it acts. There arethree distinct forces present in this contact zone which act upon theagglomerated suspension. These forces are a fluid dynamic shear force, afluid dynamic compressive force and an electrical field force if a fieldis applied. The theory is that when the threshold failure stress isexceeded, the agglomerate particle is severed and particle (color)separation is obtained. This dismembennent is a structural failure thatmust subscribe to one of the general prevailing failure theories forsolid bodies. The forces normally acting upon the particles in thesuspension during imaging are too small to affect particle structuralfailure. In fact, only in the presence of the fluid dynamic stresses andcompressive forces, is there enough of a resultant stress available tosever the agglomerate particles and produce improved imaging and colorseparation in a polychromatic system. Since this theory has beenexperimentally shown to be operative and to create both better colorseparation and image quality, there is, of course, no intention to limitthe invention to this theory of operation which is only given to clarifythe results obtained with the method and structure disclosed herein anddescribing this invention.

EXPERIMENTAL RESULTS The following examples were carried out in anapparatus of the general type illustrated in FIG. 1. The stressingroller 30 was approximately 2.5 inches in diameter. The imaging velocitywhich is equal to the translational velocity of electrode roller 16 wasabout 1.5 inches per second. The plate employed was approximately 4inches by 4 inches andwas exposed to the light intensity of 200-300 footcandles. A suspension of 845 grams per liter of the'indicated particleswere held in the suspension having mineral oil for a carrier. Themagnitude of the applied potential between the two electrodes duringimaging was 2,000-4,000 volts. The pigments which were present asparticles in the suspension were ground in a ball mill for 48 hours toreduce the size to make a morestable dispersion toimprove the resolutionof the final images.

. The pigments themselves were as follows:

Monolite Fast Blue GS (cyan) mixture of the alpha and bet metal freephthalocyanine Arnold Hoffman Co.

Watchung Red .8 (magenta) (II No. 15865 1-(4- methyl-5-chloroazobenzene'-'2'-sulfonic hydroxy-3-naphthoic acid-Made by DuPont.

Yellow pigmenthaving the following structure and I prepared inaccordance with application Ser. No. 421,281 filed on Dec. 28, 1964 nowUS. Pat. No. 3,447,922 N-2"-pyridyl-8,13-dioxodinaphtho-(2, l- 2 ,3)-furan-6-carboxamide. I

Exposure was made with a 2,800-3,400 K lamp through a continuous toneneutral density gray scale laminated over several primary color andneutral interference filters to make a color wedge to measure thesensitivity of the suspension.

Besides the above parameters the stressing roller 30 velocity was 131inches per second relative to the electrode roller 16 velocity and inthe direction shown in FIG. 1. The stressing roller had a bias of lvolts relative to the electrode roller 16. The maximum hydrodynamic inkpressure developed was l5 psi. The following results were observed byillumination with a tungsten lamp through the transparency test subjectreferred to above. A representative set of date comparing optimizedresults with the stressing roller and results obtained without thestressing roller follows; A D= (analytical density of stressexperiment)-(analytical density of non-stress control):

Changes in Analytical Density of Component A change in density wasnoticed with a relative velocity' between the stressing roller and theblocking electrode roller 16 of as low as 20 inches per second with acharge differential of .l00 volts from'the stressing roller relative tothe electrode roller and a hydrodynamic ink pressure of 5 psi.

Improvements in the image density were also observed with a relativevelocity difference. between the stressing roller and the electroderoller 16 of 260 inches'per second with a'bias difference of -600 voltsfrom the stressing roller relative to the electrode roller 16 and ahydrodynamic ink pressure of 30 psi.

The above mentionedparameters are not meant to be critical and some ofthe parameters noticed experimentally were not necessarily limits on themethod or system. Some of the results were reached only because of thelimitations of the mechanisms used for the experiments and should not beconsidered critical limitations above or below which no beneficialeffect will be noted with a particular suspension.

Several assumptions are made in order to generalize the application offorces to particle suspensions different from the one tested. Basicallythe internal stresses involved in forming and holding agglomeratestogether are very small .and this is what must be overcome to achieveparticle separation. Even in particle suspensions other than the onetested, the forces maintaining interparticle attraction are relativelysmall compared to the forces exerted by the structure herein described.Because of the great variablity permissible with the structure andmethod used herein, the shear stress may be multiplied by 100x and morewithout greatly affecting the system. Therefore with any givensuspension, even one with interparticle holding forces more than 100xgreater than the suspension tested, the proper stress forces to break upthe agglomerates can be determined and the structures and methods hereindisclosed can be used to generate those forces.

The theory herein is applicable to particle suspensions forelectrophoretic imaging or for inking or for any other use where smallindividual particles are desired for better performance thanagglomerates of several particles.

MATHEMATICAL MODEL particle structures and therefore improve the use ofthe suspension in imaging systems, color separation systems, and/or inuniform layer applications forv such systems as inking of presses.

The apparatus and methods described embodying this invention exertexternal forces upon the agglomerated particles sufficient to raise thestress level within the agglomerated particle to the critical valuenecessary for structural separation of the agglomerate into discreteparticles.

The values necessary for breaking up the agglomerated particles in orderto improve both imaging and color separation in multi-color systems orimaging inmonochromatic systems are determinable based on severalvariables within the imaging apparatus and the suspension itself. Thefollowing mathematical model describes the necessary parameters in orderto achieve the desired results described herein. Relative to this modelattention is drawn to FIGS. 9 and 10.

FIG. 9 is an illustration of an agglomerated particle assumed to bewithin the contact zone such as zone 42 of FIG. 2 between the stressingroller and the electrode with which it operates. FIG. 9 also showsrepresentative forces acting upon the particle held within thesuspension. The forces marked F represent the electrical field forceacting on the particleQthe forces marked p represent the pressure actingon the agglomerate which is equal to the pressure within the suspensionin the vicinity of the agglomerate, and T represents the shear stressacting on the agglomerate or the shear stress within the suspension atthe contact zone.

FIG. 10 represents an elemental cube taken within the agglomeratedparticle to represent the stresses on that particle in simplifiedmathematical terms. Here the stresses represented by a p is the pressureexerted through the suspension on the particle and therefore theelemental cube within the particle. The electrical stress acting uponthe particle is represented by q and the shear stress on the elementalcube is represented by the T.

Viewing the elemental cube as existing within an agglomerate in thecontact zone and considering the shear stress on the agglomerate andwithinthe suspen. sion in that zone as caused by a uni-directionalviscous flow, the shear stress T=,u. dV/dy where 1.4. is the apparentviscosity of the suspension, and dV/dy is the shear rate within thefluid. The force F shown in FIG. 9 which acts upon the particlegenerates an electrical stress q such that q F /D where D is acharacteristic length of the particle. This stress is uni-directionalbecause of the characteristics of the electrical field in the contactzone. I

From the illustrations of FIGS. 9 and 10 the following componentstresses within the agglomerate can be found. It is assumed that thevalues of the stress are maximal at the intersection of the verticalcenter line of the stressing mechanism and the tangential plane of theelectrode which itcontacts.

14 within the particle.

When the stress component which characteristically determines structuralfailure of the agglomerate particle, such as, for example, the shearstress, exceeds the critical level for agglomerate fracture, theagglomerate separates into discrete particles. These discrete particlescan then function separately in the manner described herein to functionbetter within the system in which they are employed.

Although the mathematical model explaining the failure theory of theagglomerated particles is based on maximum shear stress theory offailure, the experimentally provable agglomerate failures could beexplained by other failure theories such as a principal stress theory.Nevertheless apparatus or methods employed for breaking agglomerates insuspensions such as those described herein that come within the shearstress mathematical model are of the type contemplated by thisinvention, even if actual agglomerate failure is explainable by someother theory.

The theories expounded above and experiments conducted in accordancewith these theories indicate another very useful benefit from theoverdriven high speed roller such as shown in FIG. 1. When operating inthe configuration shown in these figures, a relatively uniform thinlayer suspension is left on the member adjacent the high velocity rollerafter it has been contacted by the high velocity roller. There is enoughuniformity in the layer remaining on the member that the method oramount of manual application of the suspension does not substantiallyaffect the experiments such as those described above.

While this invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth; and thisapplication is intended to cover such modifications or changes as maycome within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:

1. A suspension stressing process for breaking interparticle bonds inorder to separate agglomerated particles within suspensions comprisingthe steps of:

a. providing a layer of suspension of up to about one mil thicknesscontaining agglomerated particles;

b. applying a fluid dynamic compressive stress to said suspension;

c. simultaneously applying a fluid dynamic shear stress to saidsuspension by contacting said suspension with two surfaces, saidsurfaces having a surface velocity relative to each other of betweenabout 20 and about 300 inches per second; and,

d. simultaneously applying across said suspension an electrical field,said shear stress, compressive stress and electrical field beingsufficient to cause said agglomerates to break apart.

2. The method of claim 1 wherein said electrical field is in the rangeof from about 50 to about 600 volts.

3. The method of claim 1 wherein said shear stress on the particleswithin the suspension is from about 5 to about 30 pounds per squareinch.

4. The method of claim 1 wherein said compressive stress is from about 5to about 30 pounds per square inch.

Patent No. 3,833,493 Dated September 3, 1974 h fl Edwin Zucker It iscertified that error appears in the above-identified patent and thatsaid Letters Patentare hereby corrected as shown below:

Column 2, line 46, delete "over-coated" and insert -overcoated--.

Column 4, line 31, delete "of" Column 10, line 4 1, delete "stess andinsert stress--.

Column 11, line 28, delete "bet" and insert -beta--.

Column 12, line 28, delete "variablity" and insert i --variability-.

i Signed and sealed this 19th day of November 1974.

(SEAL) I Attest:

McCOY M. GIBSON- JR. I c. MARSHALL DANN Attesting Officer Commissionerof Patents FORM PC4050 (w'ss) uscoMM-oc 60376-P69 v A ".5. GOVERNMENTPRINTING DFFICE: i9! 0-355-33.

2. The method of claim 1 wherein said electrical field is in the rangeof from about 50 to about 600 volts.
 3. The method of claim 1 whereinsaid shear stress on the particles within the suspension is from about 5to about 30 pounds per square inch.
 4. The method of claim 1 whereinsaid compressive stress is from about 5 to about 30 pounds per squareinch.